Research Document - 2011/111

Stock assessment for Pacific ocean perch (Sebastes alutus) in Queen Charlotte Sound, British Columbia

By A.M. Edwards, P.J. Starr, and R. Haigh

Abstract

Pacific ocean perch (Sebastes alutus, POP) is a commercially important species of rockfish that inhabits the marine canyons along the coast of British Columbia. The status of POP in Queen Charlotte Sound, British Columbia, is assessed here under the assumption that it is a single stock harvested entirely in Pacific Marine Fisheries Commission (PMFC) major areas 5A, 5B and 5C. This stock has supported a domestic trawl fishery for decades and was heavily fished by foreign fleets from the mid-1960s to mid-1970s.

We used an annual catch-at-age model tuned to three fishery-independent trawl survey series, annual estimates of commercial catch since 1940, and age composition data from two of the survey series (8 years of data) and the commercial fishery (29 years of data). The model starts from an assumed equilibrium state in 1940, and the survey data cover the period 1967 to 2010 (although not all years are represented). The two-sex model was implemented in a Bayesian framework (using the Markov Chain Monte Carlo procedure) under four scenarios, in which natural mortality, M, and steepness of the stock-recruit function, h, were fixed or estimated.

All four model runs suggest that strong recruitment in the early 1950s sustained the foreign fishery, and that a few strong year classes spawned in the late 1970s and 1980s sustained the domestic fishery into the 1990s. At the Pacific Region Centre for Science Advice review meeting, participants endorsed the two runs that estimated h as being equally plausible (one is termed “Estimate M & h” and the other “Estimate h”).Participants rejected the other two runs for provision of advice to management.

The spawning biomass (mature females only) at the beginning of 2011 is estimated to be 26% (12%, 43%) of unfished biomass (median and 5th and 95th quantiles of the Bayesian posterior distribution) for run “Estimate M & h”, and 14% (8%, 24%) for run “Estimate h”. The estimated spawning biomass at maximum sustainable yield, B_msy, is estimated to be 25% (17%, 35%) of unfished biomass for run “Estimate M & h”, and 24% (16%, 32%) for run “Estimate h”.

Advice to managers is presented as decision tables that provide probabilities of exceeding limit and upper stock reference points for five-year projections across a range of constant catch scenarios for both model runs. The DFO provisional ‘Precautionary Approach compliant’ reference points were used, which specify a ‘limit reference point’ of 0.4B_msy and an ‘upper stock reference point’ of 0.8B_msy.  The estimated spawning biomass at the beginning of 2011 has a 0.96 or 0.82 probability (runs “Estimate M & h” and “Estimate h”, respectively)of being above the limit reference point, and a 0.68 or 0.24 probability of being above the upper stock reference point. Five-year projections to 2016 indicate that the spawning biomass has probabilities of 0.91 or 0.57 of remaining above the limit reference point, and of 0.63 or 0.15 of remaining above the upper stock reference point, if catches average 3,500 t/y, which is the average level of removals from 2006–2010. 

We note that the definitions of the PMFC areas differ from the Groundfish Management Areas (GMAs) used by the GMU. Based on the most recent five years, the combined POP landings in GMAs 5AB and 5CD were about 5% greater than the landings in the combined PMFC areas 5ABC. Current POP Total Allowable Catches are 2,070 t for GMAs 5AB and 2,118 t for GMAs 5CD.

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